Perkins 4-108 cooling

[nimbusgb]

Beneteau Oceanis 390. Perkins 4108, 5000 hours and AFAIK the prop is the same one fitted to the boat 25 years ago. 3 blade fixed pitch (unknown) but 5 knots at 1900 rpm with a fairly foul bottom. Gearbox is near as damnit 2:1. The boats documents document list the engine at 50hp ( the 4108 had lots of variants I know )

I cant verify the indicated RPM is right but it 'sounds' about right. I have an optical rev counter to use next time I'm aboard .

The engine is fitted with an impeller housing with two impellers with a dividing plate between them, essentially two pumps but the dividing plate does have a small equalisation hole between the two chambers. The rear chamber takes raw water from the seacock and pumps it into the heat exchanger mounted on the top of the engine, the overflow from that goes to the exhaust injector elbow. The forward chamber takes water from somewhere ( It MUST be a takeoff from the heat exchanger surely ) and pushes it to the stern gland ( rubber boot fed with 'bearing' water ). The normal engine pump also drives a calorifier circuit, that water gets good and hot.

Last trip out to the boat I fitted a large intake strainer with a clear lid between the seacock and the first pump. There is a filter with a very fine stainless mesh between the second pump and the stern gland.

Engine gets hot if run above about 1900 rpm. Evidence is first a little steam out of the exhaust and then eventually ( 10 mins? ) a high temperature warning and an indicated 100 degrees C.

I think that there may be too little water being delivered to the heat exchanger, the amount coming out of the exhaust seems to be somewhat less than other boats. How much water is needed? Running the motor ashore sucking intake water from a bucket would allow me to measure the flow I suppose ( any thermodynamics engineers here? )

Shouldn't the engine be able to run significantly higher than 2000 rpm? The manual I have off the internet suggests max power ( 46hp ) at around 4000 and max continuous (37) at 3000. That sounds about right for this kind of diesel but no way is this one going to run that fast form more than a few minutes without overheating!

My geuss is that there has been no significant maintenance on this engine in several thousand hours

Does the engine need de scaling and de coking. ( think I know the answer to this one! )

TIA
Ian

 

[VicS]

The appearance of a little steam from the exhaust indicates that the raw water flow is too low to carry away all the heat transferred to it via the heat exchanger.

The first things to do are investigate the raw water circuit fully for any obstructions from the inlet through the heat exchanger to the exhaust injection point. including the condition of the hoses and to overhaul the raw water pump if it has not been done recently. The pump bodies can wear, the cam plate can wear down, in addition to the impeller.

Although general scaling of the heat exchanger can cause over heating it would not result in the exiting cooling water being hot but localised scaling could be restricting the flow.

So pump, heat exchanger (raw water side) injection point and the rest of the raw water circuit, looking for whatever is causing the flow to be low.

 

[Caer Urfa]

Only a thought!
I spent weeks trying to find out why my Perkins 4108 was showing overheating signs on my last boat, in the end I found some plastic bag traped between the raw water inlet (below water) in the plastic pipe BUT before the actual strainer/filter.
Take the internal fine strainer out and blast water DOWN the intake and 'look overboard' to see if anything blows out.
Worth a try and takes a few minutes!
Good luck
Mike

 

[milfordman]

I have a perkins 4-108 in an ebbtide 33. From what I recall from my manual the figures of max 4000 rpm and 3000rpm giving 37 HP is correct. In my boat with a 19 inch prop I get 5Knots @ 1300 rpm in calm to moderate condition. I can get 6.5 knots @2500 rpm. However I prefer the lower figures for good fuel economy and a quiet life.

I have run the engine as you propose, with the inlet pipe disconnected from the sea cock, and put in a bucket which was continualy replenished by a hose pipe. I did not record times to empty the bucket. However my recollection is that the engine could not be run at more than 1200 rpm as the hose pipe supply could not keep up. I would estimate that the 2 gallon bucket would fill up in approx 15 to 20 seconds, that would give a flow rate of 6 - 8 gallons per minute@ 1200 rpm. I know these figures are a bit of a guestimate. But they will give you a ball park figure to work from. I have a temperature gauge installed and the temperature usually runs at 140 to 165 deg F. P.S. I have a conventional jabsco pump pumping seawater through heat exchanger then injected into the exhaust elbow.

I agree with earlier sugestions. Check pump for wear. Belt tension. Collapsing pipe hoses. Blocked heat exchanger.

 

[nimbusgb]

Thanks for the suggestions everyone.

Vics - thanks, good points, as you say the heat is getting to the exchanger.

I think I can discount a blockage in the intake between the hull and the new strainer since I did run the engine sucking from a bucket. The flow rate when I did this was about the same as when running afloat and way below what milfordsam achieved so I am going to have to look upstream from there.

The internals of the pump look good and both impellers have been replaced and no hoses are collapsed.

It strikes me that poor sealing when assembling the pump could allow air to be sucked in rather than water. How do people recommend sealing the pump gasket flange?

BTW just found a gem of an article on the Jabsco website

which says "Engines with indirect cooling require more flow: about 65-70 litres (14.5 - 15.5 gallons) per minute for each 75kW (100bhp) of engine power."

So at 50 hp I'd need say 35 l/m, that's at around 4000 rpm so at 2000 rpm I should be looking for about 17 l/m and at 1000 8 to 9 l/m - that should be easy enough to check and I think I'm a little shy of that

 

[galeus]

a problem I once had was the wear plate at the back of the water pump,which would result in a lack of flow,not all pumps have one and in my case necessitated a new pump(with a wear plate).

 

[nimbusgb]

i have just purchased a genuine workshop manual for the 4108 on eBay for £3 hopefully it will tell me how to check the wear plate!

 

[Pete735]

If by the wear plate you mean the plate that is removed to change the impellor, then if you remove it, wear will be obvious on it. Don't forget, these plates are reversable, so just turn it round and use outside face as inner face - assuming it's not already been done. If needed, Jabsco sell the end plate as a separate piece.

Having said that, unless the actual impellor is missing a blade or two and this plate is worn as well, I would be looking for a blockage somewhere. I also agree with Milfordsam's comments on water throughput, at 1500rpm, my 4108 will empty an average bucket in a few seconds - I was very surprised when I tried this last year.

Good luck

 

[nimbusgb]

I have a double impeller pump so there is a cover chamber, and an intermediate plate and a fixed rear chamber, impeller in each chamber. I might have to have a look at getting the intermediate plate skimmed nice and flat, seem to remember that it it was showing the effects of 5000 plus hours somewhat!

 

[pelissima]

Had similar prob w 4108. What you think you know at the end, is accurate. Do a proper descaling and open up the exchanger to check the pipes are clean.

 

[pcatterall]

Recent thread on this I think. My 4108 overheat problem was in the heat exchanger ( just gunged up) but I think that the very heavily scaled up pipe elbows started the rot by slowing down the amount pumped through ( they were reduced to about 25% 0f section. Strangly I cannot see any difference in the amount pumped out.

 

[branko]

Continue to Vics discussion check also Thermostat if it exist!